Islay whisky making

Distilling on Islay Casks

By law, until spirit has spent three years in an oak cask, it cannot legally be called whisky. This process of ageing allows numerous changes to happen in the new make spirit, creating the complexity and flavour that characterise Scotch single malt whisky.

The origin of the wood used to make a cask can also have a large influence on the final flavour of the whisky. Oak from the USA is the most common to be used in Scotland, with European oak second.

Casks are created from flat boards known as staves. These are cut and ‘seasoned’ before being used to build casks. This process is one of drying, dropping the moisture level of the oak down to 12-16%, depending on where the seasoning takes places. It can be done in kilns or by leaving the boards outside – kilned oak generally governs a lower price than outdoor seasoned. Once the boards are ready they are cut into straight staves, and are fashioned into circular vessels, held together with a hoop and open at one end. Through a process of heating and bending, the staves are pulled into a traditional barrel shape.

Casks vary in size from the very small to the very large, with a legal maximum of 700 litres. The most commonly used are barrels, hogsheads and butts, holding approximately 200, 250 and 500 litres respectively. The larger the cask, the smaller the ratio of wood area to liquid volume and thus the smaller potential for the wood to have an influence on the spirit. 

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While some new oak casks are used in Scotch whisky production, the vast majority have held some kind of liquid before. The most commonly used cask in Scotland is an American oak ex-bourbon cask – these are casks that have already been used to mature bourbon whiskey. Bourbon regulations require that a new cask is used every time, providing a ready supply of casks for use in Scotland.

Ex-sherry casks are also used but since regulations forbidding the shipment of sherry in cask came into force in the 1980s, they have become rarer and much more expensive. Sherry casks can be made from both European or American oak, and may have either been used for the production of actual bottled sherry or simply filled with sherry to season the wood before being used to mature whisky.

The previous fill of a cask will have both added and extracted compounds from the wood, and will influence the flavour of the spirit matured in the cask.

European and American oaks also have differing structure within the wood, leading to different character being imparted to the spirit: American oak has a tighter grain, giving a lighter colour, and more coconut and vanilla flavour; European oak has wider grain, giving a darker colour, more tannic notes and weightier flavours.

As casks are reused, the influence of the original fill and the oak itself will decrease, as the flavour compounds are stripped from the wood. However, each occupant will leave an impression on the cask, making every one unique.

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